Heat exchangers are routinely employed within the automotive industry, such as in the form of radiators for cooling engine coolant, condensers and evaporators for use in air conditioning systems, and heaters. In order to efficiently maximize the amount of surface area available for transferring heat between the fluid within the heat exchanger and the environment, the design of the heat exchanger is typically a tube-and-fin type, which contains a number of tubes that thermally communicate with high surface area fins. The fins enhance the ability of the heat exchanger to transfer heat from the fluid to the environment, or vice versa.
To further enhance heat transfer efficiencies, the tubes may be in the form of "microtubes." A microtube is generally distinguishable from a standard heat exchanger tube by having a relatively small and flat cross-section, for example, on the order of about 1.7 by about 25 millimeters, and very thin walls, for example, on the order of about 0.2 to about 0.4 millimeter. As such, microtubes offer a larger surface area for a given cross-sectional area, with enhanced thermal conduction through the tube wall due to the wall being significantly thinner than that of a standard heat exchanger tube.
Increasingly, heat exchangers used in the automotive industry are being formed from aluminum alloys for the purpose of minimizing the weight of automobiles. Conventionally, such heat exchangers are constructed using one of several methods. One method utilizes mechanical expansion techniques and has been traditionally used for mass-producing radiators. Mechanical expansion techniques rely solely on the mechanical joining of the components of the heat exchanger to ensure the integrity of the heat exchanger, such as the joining of the tubes to the fins. Advantages of this method of assembly include good mechanical strength and avoidance of joining operations which require a furnace operation, while disadvantages include inferior thermal performance and relatively large size.
To overcome the disadvantages of the mechanical expansion-type heat exchangers, heat exchangers are increasingly being formed by a brazing operation. Such methods generally entail fixturing the individual components of a heat exchanger together, and then permanently joining the components with a suitable brazing alloy during a furnace operation to form the heat exchanger assembly. Generally, brazed heat exchangers are lower in weight and are better able to radiate heat as compared to mechanical expansion-type heat exchangers. An example of such a heat exchanger is referred to as the serpentine tube-and-center type, which involves one or more serpentine-shaped tubes which traverse the heat exchanger in a circuitous manner. The serpentine-shaped tubes are brazed to a number of high surface area finned centers to enhance heat transfer to the environment through thermal convection. Another type of heat exchanger is referred to as the headered tube-and-center type, or parallel flow type, and involves a number of parallel tubes which are brazed to and between a pair of headers. Finned centers are brazed between each adjacent pair of tubes for heat transfer by convection. Vessel-like members are placed at each header to form tanks therewith which are in fluidic communication with the tubes.
Brazing of aluminum-base components to form a heat exchanger is complicated by the inherent presence of an aluminum oxide layer on the surface of such components when exposed to an atmosphere containing oxygen. The oxide layer cannot be readily wetted, such that the formation of a strong metallurgical bond between a braze alloy and the aluminum members is significantly inhibited. To overcome such difficulties, one brazing technique in practice involves an inert atmosphere furnace operation. To destroy and remove the oxide layer, the assembly or its individual components are typically sprayed with or dipped into a water-based flux mixture prior to the brazing operation. The assembly is then dried to evaporate the water, leaving only the powdery flux solids on all of the external surfaces of the assembly. During brazing, the flux removes the oxide layer so as to expose the underlying aluminum surface to the braze alloy.
The brazing operation is complicated by the numerous brazements required, particularly when assembling a headered tube-and-center type heat exchanger, wherein each tube must be brazed to both headers and its corresponding finned centers during a single brazing operation. Typically, the brazements are achieved by employing an aluminum alloy brazing stock material to form the tubes, headers and/or finned centers. The aluminum alloy brazing stock material consists, for example, of an appropriate aluminum alloy core which has been clad on at least one side with an aluminum-base brazing alloy. Generally, the brazing alloy has been provided on both surfaces of the finned centers and on only the external side of the header, i.e., the side through which the tubes are inserted.
The cladding layers are generally an aluminum-silicon eutectic brazing alloy which is characterized by a melting point of about 575.degree. C. to about 610.degree. C., such that the brazing alloy has a lower melting temperature than that of the core aluminum alloy, which is typically at least about 630.degree. C. The brazing operation involves carefully raising the temperature of the assembly such that only the clad layers of brazing alloy melt during the brazing operation. The brazing alloy then flows toward the desired joint regions and, upon cooling, solidifies to form the brazements.
Conventionally, it is known to provide the brazing alloy as 1) a foil which is brazed to the extruded tubes of a tube-and-center type heat exchanger, 2) a molten coating which is deposited onto the extruded tubes, or 3) a liner on an ingot which is hot and cold rolled to produce a silicon-clad aluminum alloy foil used to form the finned centers and headers of a headered tube-and-center type heat exchanger or finned centers of a serpentine tube-and-center type heat exchanger. A shortcoming of the first two above-described processes, i.e., the brazed foil and molten coating processes, is that there are two fluxing operations required: the first to adhere the brazing alloy to the tube's aluminum alloy core, and a second to braze the tubes to the finned centers during the braze furnace operation. The need for two fluxing operations is disadvantageous in that the additional flux, including its application and removal, add costs to the final assembly. The additional flux also aggravates the tendency for the flux to corrode the interior of the furnace, resulting in additional maintenance and repair of the furnace.
Another disadvantage with the brazed foil and molten coating processes is that the silicon within the brazing alloy tends to diffuse into the aluminum alloy core at the elevated temperatures required for the brazing operation. As a result, the corrosion resistance of the brazing alloy is reduced and, due to the reduced silicon content in the brazing alloy, the furnace temperatures required to melt the brazing alloy are higher.
The general practice of cladding the aluminum alloy core with an aluminum-silicon brazing alloy also tends to be disadvantageous in that the silicon content of the clad brazing alloy may vary significantly. For every one weight percent variation in silicon within the brazing alloy, the melt temperature of the brazing alloy can vary by about 10.degree. F. This variability in silicon content significantly complicates the process control for the subsequent furnace braze operation.
A solution to the above problems is disclosed in U.S. Pat. Nos. 4,615,952 and 4,891,275 to Knoll, which involves a continuous coating process, wherein a zinc-base alloy is substituted for the conventional aluminum-silicon alloy. In particular, Knoll teaches a novel process by which the zinc-base alloy can be deposited on the surface of an extruded aluminum alloy profile, such as a tube for a heat exchanger, so as to serve as a soldering or low temperature brazing material when properly melted during a furnace operation. The coating process is conducted immediately after the aluminum alloy tube is extruded and within an inert atmosphere, such that the formation of an aluminum oxide layer is inhibited. As a result, the zinc-base alloy is able to bond to the surface of the aluminum alloy tube without the use of a flux. The aluminum alloy tubes may then be soldered or brazed to form a heat exchanger, with the zinc-base alloy coating serving as the brazing material. An additional benefit associated with the processes taught by Knoll is that the zinc-base alloy coating improves the corrosion resistance of the heat exchanger formed therewith, not only by minimizing the use of flux, but also because the zinc serves as a sacrificial anode, thus improving the corrosion performance of the heat exchanger through the suppression of pitting.
It would be advantageous to provide further improvements in coating processes for the coating of aluminum alloy profiles, such as a tube or microtube of a tube-and-center type heat exchanger, with a zinc-base alloy, so as to eliminate the requirement for an aluminum-silicon clad brazing alloy for purposes of soldering or brazing the tube. It would also be advantageous that such an improved process be sufficiently versatile so as to permit the deposition of the zinc-base alloy coating after an aluminum oxide layer has formed on the tube. It would be additionally desirable if the improved method were capable of forming a zinc alloy coating on tubes and microtubes used to form a heat exchanger, such that the coating thickness could be closely controlled to achieve a minimal thickness for a particular application, so as to minimize the weight and material used to form the heat exchanger.